一、原型和原型物件

1. 函式的原型prototype：函式才有prototype，prototype是一個物件，指向了當前建構函式的引用地址。

2. 所有物件都有__proto__屬性， 所有的__proto 指向改物件的原型物件（注意：proto前面是兩個）

關係示意圖：

虛線代表：指標
實線：prototype原型物件

例項

    function A() {
        this.a = 1;
    }
    var a = new A();
    var obj = {};
    
    console.log(a.constructor == A); //例項中無constructor屬性，在原型中存在，因為指標連結到a.__proto，輸出原型的constructor屬性A
    
    console.log(a.__proto__ === A.prototype);  //ture 例項的__proto__指向建構函式的原型prototype
                                           
    console.log(A.__proto__ === Function.prototype);//true 建構函式的__proto__指向 Function的原型prototype
    console.log(A.prototype.constructor === A); //true 建構函式原型prototype的constructor屬性，指向建構函式
    
    console.log(A.prototype.__proto__ === Object.prototype); //所有的prototype都是Object的例項
    
    console.log(Object.prototype.__proto__ === null); //Object.prototype.__proto__ 是原型鏈的唯一出口
    console.log(typeof null); //"object"  null是一個空物件指標
 

二、原型的建立方式

2.1 單獨新增屬性方法

    function Person(){
    }
    
    Person.prototype.name = "Nicholas";
    Person.prototype.age = 29;
    Person.prototype.job = "Software Engineer";
    Person.prototype.sayName = function(){
        console.log(this.name);
    };
    
    var person1 = new Person();
    person1.sayName(); //"Nicholas"
    
    var person2 = new Person();
    person2.sayName(); //"Nicholas"
    //true  訪問的都是建構函式原型prototype的方法
    console.log(person1.sayName == person2.sayName); 
 

2.2 物件字面量方法

 a. 採用{}方式

    function Person(name, age, job) {
        this.name = name;
        this.age = age;
        this.job = job;
        this.friends = ["Shelby", "Court"]; //引用型別屬性放在建構函式中
    }
    
    Person.prototype = {
        constructor: Person,
        sayName: function () {
            alert(this.name);
        }
    };
    
    var person1 = new Person("Nicholas", 29, "Software Engineer");
    var person2 = new Person("Greg", 27, "Doctor");
    
    person1.friends.push("Van");
    
    alert(person1.friends); //"Shelby,Court,Van"  
    alert(person2.friends); //"Shelby,Court" 
    alert(person1.friends === person2.friends); //false
    alert(person1.sayName === person2.sayName); //true
 

 b. 建構函式和原型結合
  建構函式定義屬性，原型模式定義方法和共享的屬性

 function Person(name, age, job){
        this.name = name;
        this.age = age;
        this.job = job;
        this.friends = ["Shelby", "Court"];  //引用型別屬性放在建構函式中
    }
    
    Person.prototype = {
        constructor: Person,
        sayName : function () {
            alert(this.name);
        }
    };
    
    var person1 = new Person("Nicholas", 29, "Software Engineer");
    var person2 = new Person("Greg", 27, "Doctor");
    
    person1.friends.push("Van");
    
    alert(person1.friends); //"Shelby,Court,Van"  
    alert(person2.friends); //"Shelby,Court" 
    alert(person1.friends === person2.friends); //false
    alert(person1.sayName === person2.sayName); //true
    

2.3 class類

    class A {
        constructor(name) {
            //建構函式
            this.name = name; //給新的物件新增一個name屬性
        }
    
        // sayName相當於 A.prototype.sayName = function(){return this.name}
        sayName() {
            return this.name;
        }
    }
    
    var a = new A('zhangsan');
    console.log(a);

三、 原型物件屬性為引用型別

    function Person() {}
    
    Person.prototype = {
        constructor: Person,
        name: "Nicholas",
        age: 29,
        job: "Software Engineer",
        friends: ["Shelby", "Court"],
    
        sayName: function () {
            console.log(this.name);
        }
    };
    
    var person1 = new Person();
    var person2 = new Person();
    
    person1.friends.push("Van");
    
    console.log(person1.friends); //"Shelby,Court,Van"
    console.log(person2.friends); //"Shelby,Court,Van"
    console.log(person1.friends === person2.friends); //true